The present invention relates generally to synchronizers, and more specifically to a synchronizer particularly suitable for command signals of computers.
With a synchronizer implemented with at least one D-type flip-flop, the application of an asynchronous data will cause the flip-flop to assume what is called a "metastable" state if the transition point of the data coincides with the transition point of the sampling pulse with a frequency at least twice as higher than the maximum frequency of the asynchronous data (Nyquist criterion). If this type of synchronizer is employed in computers for synchronizing asynchronous command pulses propagating through an asynchronous bus system with the clock timing of a synchronous bus system, the sampling frequency of the clock timing must be sufficiently high so that the pulse of the least possible duration of the asynchronous command signal may be sampled. Since the period of the inactive state of the asynchronous command signal cannot be stretched while the period of the active state can be altered using a known wait control technique, the sampling frequency must be chosen so that the inactive state of least duration may be sampled. However, the asynchronous command signal may contain pulses of very short duration due to the introduction of newly developed high-speed circuitry to the asynchronous system and may escape the sampling points of the synchronous system if the clock frequency of the latter remains unchanged, resulting in the violation of the Nyquist criterion.